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|Section2= |Section7= }} Cholesterol, from the Ancient Greek ''chole-'' (bile) and ''stereos'' (solid) followed by the chemical suffix ''-ol'' for an alcohol, is an organic molecule. It is a sterol (or modified steroid), a lipid molecule and is biosynthesized by all animal cells because it is an essential structural component of all animal (not plant or bacterial) cell membranes that is required to maintain both membrane structural integrity and fluidity. Cholesterol enables animal cells to not need a cell wall (like plants and bacteria) to protect membrane integrity and cell viability, thus are able to change shape and move about (unlike bacteria and plant cells which are restricted by their cell walls). In addition to its importance within cells, cholesterol also serves as a precursor for the biosynthesis of steroid hormones, bile acids, and vitamin D. Cholesterol is the principal sterol synthesized by animals. All kinds of cells in animals can produce it. In vertebrates the hepatic cells typically produce greater amounts than other cells. It is almost completely absent among prokaryotes (bacteria and archaea), although there are some exceptions such as ''Mycoplasma'', which require cholesterol for growth. François Poulletier de la Salle first identified cholesterol in solid form in gallstones in 1769. However, it was not until 1815 that chemist Michel Eugène Chevreul named the compound "cholesterine".〔Chevreul (1816) "Recherches chimiques sur les corps gras, et particulièrement sur leurs combinaisons avec les alcalis. Sixième mémoire. Examen des graisses d'homme, de mouton, de boeuf, de jaguar et d'oie" (Chemical researches on fatty substances, and particularly on their combinations o filippos ine kapios with alkalis. Sixth memoir. Study of human, sheep, beef, jaguar and goose fat), ''Annales de Chimie et de Physique'', 2 : 339-372. From (page 346 ) : "Je nommerai ''cholesterine'', de χολη, bile, et στερεος, solide, la substance cristallisée des calculs biliares humains, ... " (I will name ''cholesterine'' — from χολη (bile) and στερεος (solid) — the crystalized substance from human gallstones ... )〕 ==Physiology== Since cholesterol is essential for all animal life, each cell synthesizes it through a complex process beginning with mevalonate pathway and ending with the 19 step conversion of lanosterol to cholesterol. Particularly high levels of fats (including cholesterol) in the circulatory system, depending on how they are transported within lipoproteins, are strongly associated with the progression of atherosclerosis (arteriosclerotic vascular disease). A human male weighing 68 kg (150 lb) normally synthesises about 1 g (1,000 mg) per day, and his body contains about 35 g, mostly contained within the cell membranes. Typical cholesterol dietary intake for a man in the United States is 307 mg, which is above the upper limit recommended by the Dietary Guidelines Advisory Committee.〔 〕 Most ingested cholesterol is esterified, and esterified cholesterol is poorly absorbed. The body also compensates for any absorption of additional cholesterol by reducing cholesterol synthesis.〔 〕 For these reasons, seven to ten hours after ingestion, cholesterol will show little, if any, effect on total body cholesterol content or concentrations of cholesterol in the blood. However, during the first seven hours after ingestion of cholesterol, the levels significantly increase. Cholesterol is recycled. The liver excretes it in a non-esterified form (via bile) into the digestive tract. Typically, about 50% of the excreted cholesterol is reabsorbed by the small bowel back into the bloodstream. Plants make cholesterol in very small amounts. Plants manufacture phytosterols (substances chemically similar to cholesterol produced within plants), which can compete with cholesterol for reabsorption in the intestinal tract, thus potentially reducing cholesterol reabsorption. When intestinal lining cells absorb phytosterols, in place of cholesterol, they usually excrete the phytosterol molecules back into the GI tract, an important protective mechanism. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Cholesterol」の詳細全文を読む スポンサード リンク
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